1. Field of the Invention
The present invention relates to an image lens array for a small image taking device, and more particularly to an imaging lens array consisted of three lenses that are arranged at predetermined intervals.
2. Description of the Prior Arts
An image lens array consisted of 3 lens normally can be arranged in 8 different manners to get different refracting powers, such as: A, positive/negative/positive B, positive/positive/negative C, positive/positive/positive D, positive/negative/negative E, negative/positive/positive F, negative/positive/negative G, negative/negative/positive H, negative/negative/negative. Among the above arrangements, the back focal length of the negative refracting power type lens arrays, normally E, F, G and H, are too long to minimize the whole dimension of the lens array, and it is unsuitable for small handy image taking device. Therefore, the arrangements A–D are virtually the mainstream due to facilitating minimization of the dimension of the lens array. In the prior arts, with reference to table 2, wherein the capital G represents glass lens, while P indicates plastic lens.
Column A shows 12 Japan patents, most arrangements of the first ten patents are consisted of three plastic lenses in consideration of cost, however, the telescope ratio (the ratio of the focal length to the entire optical length of these patents) is greater than 1.5, they are also unsuitable for use on the imaging taking device of the cell phone. Besides, although the conditions and requirements of incident angle of the prefix aperture are easily satisfied, its disadvantages are eccentricity and optical performance weakening of the second lens. In practical production, the refracting power of these lens arrays is difficult to be arranged.
The Japan patent No 11 in the column A is also a three plastic lenses array but with mid aperture, although mid aperture can improve the eccentricity-induced poor optical performance, the telescope ratio will be as high as 2.5.
The Japan patent No 12 in the column A is also a mid aperture type three plastic lenses array whose telescope ratio is 1.24. However, it will become dark when Fno is set at 3.5, and in practical use, it is unable to provide enough brightness. Furthermore, the small telescope ratio of will increase the incident angle up to 25 degree, so it will not be suitable for matching with CCD lens (charge coupled device).
The lens arrays as disclosed by the Japan patents in column B have a small telescope ratio, however, when the telescope ratio is too small, the back focal length will become too short, so that the lens array can't spare enough spaces for insertion of infrared filter or CCD protective glass.
Telescope ratio of the lens arrays disclosed in Japan patents No 1–3 or 6–10 stated in the column B is over 1.4, it is not handy and small enough, and the minimum telescope ratio is approximately 1.2–1.4 after insertion of the infrared filter or CCD lens.
Japan patents No 4, 5 and 11 in the column B are a three plastic lenses array, the lens array is maintained at small size when their telescope ratio is 1.2–14. f12/f<1.0 (f12 represents the combined focal length of the first and second lenses, f is the focal length of the entire lens array). As compared with the devices of the columns A, C and D, the refracting power of the second lens of column B is excessively strong, consequently, the peripheral thickness of the lens is difficult to be controlled, and eccentricity is likely to be caused during process of lens forming or assembling. The astigmatic difference of the patents nearby the middle portion of column B become large, so that, in mass production, the eccentricity of the off-axis Meridional image of the patents nearby the middle portion of column B will lead to a poor performance.
The first lens of whichever Japan Patents in column C is made of low dispersion high price glass used on tape-type image taking device, its high price is a problem, and the length of the image lens array is determined by the telescope ratio ranging from 1.4 to 1.7. Therefore, it doesn't facilitate minimization of the size of the lens array. Furthermore, the patents in column C don't have any prefix aperture type lens array.
Some lens arrays disclosed in Japan Patents in column D have the same refracting power as column A and its telescope ratio is as large as 1.54, Therefore, it doesn't facilitate minimization of the size of the lens array. Furthermore, the patents in column D don't have any prefix aperture type lens array.
Among the above-mentioned image lens arrays, the arrangement as stated in column A is commonly used, but its telescope ratio is over 1.4 and is not suitable for shortening the entire optical length. Columns B and D are more suitable shortening the entire optical length. The telescopic lens assembly normally comprises 2–3 lens and is used on image-taking lens array of silver salt camera. Since back group has negative refracting power and is difficult to be wide angled, it is unsuitable for use on the optical system whose back lens group needs to be wide-angled. Furthermore, CCD lens can not be used on the above-mentioned image-taking lens assembly due to incident angle problem.
The lens arrangements E–H are reversed telescopic lens array whose first lens has a negative refracting power, and two-lens formed VGA (video graphic array) only can be used on the 0.3–0.4 Mega pixel level digital camera and is unsuitable for use for reversed focusing and reducing the entire optical length. Therefore, lens array consisted of more than 3 lenses is not used on simply structured digital camera.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.